So-called “3-D printing” is a method of creating three-dimensional objects by depositing or forming thin layers of material in succession so as to build up the desired 3-D structure. It is sometimes called Rapid Prototyping and Manufacturing (RP&M). The process has some similarities to normal printing in that a digital representation of an object to be formed is used and each layer is formed as if it were one layer of printing, e.g. by moving some kind of printing head over a workpiece and activating elements of the printing head to create the “printing”. Various methods have been devised to create the thin layers. One technique makes use of a bath of polymerisable liquid material. A thin upper layer of the liquid is cross-linked or hardened in some way, e.g. via laser light, in a pattern which is the same as a cross-section through the object to be formed. The laser spot is moved across the surface in accordance with a digital representation of the relevant cross-section. After one layer is completed the liquid level is raised over a small distance and the process repeated. Each polymerised layer should be sufficiently form stable to support the next layer.
In another technique powder is dusted onto a substrate and the powder coalesced by some means, e.g. by heating or by the use of a liquid hardener, in accordance with the shape of the cross-section of the object to be formed. In yet another method, cross-linkable or hardenable material is deposited in the form of drops which are deposited in a pattern according to the relevant cross-section of the object to be formed. Still another method involves dispensing drops of molten material at an elevated temperature which then solidify on contact with the cooler work piece.
There are many items which can be produced by 3-D printing. Due to the fact that the materials used to form the object are subject to many limitations and are generally polymeric in nature, the final product is not very strong. Thus, 3-D printing is often used in prototyping, for example, to create a product design which can be handled or even tested for certain properties.
Printing plates are generally not made by 3-D printing techniques but usually by etching methods. Such plates are used in a variety of printing methods, such as flexographic printing, letterpress, offset or gravure printing. A simple form of a printing plate is the conventional rubber stamp.
Flexographic printing or flexography is a printing process where rubber mats or photopolymer plates and fast-drying fluid inks are used. Flexographic printing plates have the printing image in relief, which means that the image area is raised relative to the non-image area. The result is a relief plate that is capable of transferring ink from an anilox roll to a substrate. Almost any material that can run through a web press can be printed in this way, including hard-surfaced material such as acetate and other plastic films. Flexography has also been known as “aniline” printing.
Letterpress is a printing process where the image is raised as well and inked to produce an impression.
Offset printing is a method of printing in which the image is not printed directly from a plate, but is offset onto a cylinder which performs the actual printing operation. The printing plate generally has image-selective and hydrophobic regions on a hydrophilic background.
Gravure printing is a printing process where the image is etched into a plate or cylinder in the form of recesses or wells. These recesses or wells are filled with ink and the remaining surface is wiped clean, thus leaving the ink only in the recesses or wells. The image can then be printed off e.g. onto an absorbent material such as paper.
There are several additional methods of transferring an image from the printing plate onto the printing medium. For instance in tampon printing, a plate comprising an image in relief (or a negative image as in gravure printing) is inked. Afterwards, ink is transferred to a soft tampon printing head by contacting the tampon surface with the inked image. The tampon is then used to print another object, e.g. an object with an irregular surface.
In all the above printing methods, 3-dimensional printing plates are used which comprise a substrate with raised parts and recesses. In some of the printing industries, such as flexographic printing and letterpress, the raised parts are used for forming the image, while in gravure printing, the recesses form the image. In tampon printing either can be used.
The smallest individual raised portion on a flexographic printing plate relates to an isolated single pixel of an image. As the resolution of the image increases, the size of a pixel becomes smaller. Assuming that small parts of the image result in 3-D structures on the printing plate having a certain height L and a certain diameter d, one form of damage to the printing plate is Euler buckling. Euler buckling is buckling of thin column into a bow-like or wave-like shape. The critical load which can be applied before buckling is initiated varies approximately as
                              P          cr                =                                            π              2                        ⁢            E            ⁢                                                  ⁢            I                                L            2                                              (        1        )            
wherein E is Young's modulus and I is the moment of inertia. For a quadratic cross-section the value of I is proportional to the cube of the thickness—hence the danger of mechanical failure increases as a fast function of the reduction in thickness of a protrusion. Hence, problems with small parts of the image grow rapidly as the resolution increases. Confirmation of this fact can be found in that it has been known in the flexographic printing industry that small dots on flexographic printing plates (i.e. small protruding parts) tend to break off or wear easily, with image artefacts as a consequence such as discontinuities in tone gradation (near white and near black) of the printed material.
Gravure rolls are manufactured by an expensive and time consuming etching process or by means of a diamond stylus which embosses the gravure roll. Gravure printing is popular for high quality large runs, e.g. monthly journals, where the expense of the printing rolls is outweighed by the low cost per printed page and the high volumes, and where everything is planned in beforehand. However, there is an interest in making gravure plates of reasonable quality which could be used for shorter runs or for other applications, e.g. the printing of adhesives.